Dynamics of laser-induced tunable focusing in silicon

Abstract

We report here on focusing of a probe IR (λ = 1.55 μm) laser beam in silicon. The focusing is done by a second pump laser beam, at λ = 0.775 μm and 30 ps pulse width, with a donut shape that is launched collinearly and simultaneously (with some delay time) with the IR beam pulse. The pump beam pulse is absorbed in the silicon and creates, temporally, a free charge carriers (FCCs) donut pattern in the silicon. Following the plasma dispersion effect, the donut FCCs shapes a complex index of refraction pattern in the silicon that serves as a sort of dynamic GRIN lens for the probe beam due to the diffusion of the FCCs towards the donut center. This lens can be tuned to its focal point by the pump-probe delay time to reduce the point spread function (PSF) of the IR probe beam. We start seeing the focusing of the probe beam at pump-probe delay time of $$\mathrm{\Delta t }\approx 100\mathrm{ ps}$$ Δ t ≈ 100 ps . The best focusing (results in PSF $$< 2\mathrm{ \mu m}$$ < 2 μ m ) was observed at $$\mathrm{\Delta t}\approx 350\mathrm{ ps}$$ Δ t ≈ 350 ps and it slowly degrades before the FCCs full recombination at $$\mathrm{\Delta t }\sim 12\mathrm{ ns}$$ Δ t ∼ 12 ns . We propose this beam shaping method to overcome the diffraction resolution limit in silicon microscopy on and deep under the silicon surface dependent on the pump wavelength and the pulse width. We also proposed this technique for direct measurement of the FCCs dynamics.

Laser Beam Self-Focusing in Silicon at an Absorbed Wavelength by a Vortex Beam in the Same Wavelength

“…Our research aims to achieve super-resolution in semiconductors by temporally and spatially controlling their complex index of refraction. Specifically, in silicon bulk samples, we have successfully demonstrated super-resolution and focusing of an infrared (IR) probe beam with a wavelength of 1550 nm and a pulse width of 50 ps. , Our approach involves introducing a second laser beam, acting as a pump beam, which is absorbed in the silicon and temporally induces a pattern of free charge carriers (FCCs) within the sample, corresponding to the size of the pump beam spot. The FCCs generate a change in the index of refraction according to the spatial intensity distribution of the pump beam, resulting in a reduced index of refraction and increased absorption within the FCCs pattern.…”

“…In follow-up research, 7 we utilized a new laser with a narrower pulse width of 30 ps and two harmonics, employing a wavelength of 1550 nm for the probe beam and 775 nm for the pump beam. This experimental setup yielded focusing with a point spread function (PSF) of ∼2 μm.…”